Fiber Encapsulation Mechanism for Energy Dissipation in a Fiber Amplifying System

ABSTRACT

The present disclosure relates to a fiber encapsulation mechanism for energy dissipation in a fiber amplifying system. One example embodiment includes an optical fiber amplifier. The optical fiber amplifier includes an optical fiber that includes a gain medium, as well as a polymer layer that at least partially surrounds the optical fiber. The polymer layer is optically transparent. In addition, the optical fiber amplifier includes a pump source. Optical pumping by the pump source amplifies optical signals in the optical fiber and generates excess heat and excess photons. The optical fiber amplifier additionally includes a heatsink layer disposed adjacent to the polymer layer. The heatsink layer conducts the excess heat away from the optical fiber. Further, the optical fiber amplifier includes an optically transparent layer disposed adjacent to the polymer layer. The optically transparent layer transmits the excess photons away from the optical fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application claiming priorityto Non-Provisional patent application Ser. No. 15/691,917, filed Aug.31, 2017, which itself claims priority to Non-Provisional patentapplication Ser. No. 15/294,886, filed Oct. 17, 2016 and issued as U.S.Pat. No. 9,787,048 on Oct. 10, 2017. The contents of Non-Provisionalpatent application Ser. Nos. 15/691,917 and 15/294,886, as well as thecontents of U.S. Pat. No. 9,787,048, are hereby incorporated byreference.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Optical fiber lasers and amplifiers are used to radiate light atspecific wavelengths, typically at relatively high intensities. Lasersand amplifiers generally include one or more amplifier stages, eachincluding a length of active optical fiber typically coupled to one ormore pump radiation sources (e.g., pump lasers) and configured toamplify optical radiation passing through a core.

The output power of optical fiber lasers and amplifiers is beingcontinuously scaled up by designers. However, attempting to scale theoutput power can introduce problems, such as adverse energy dissipationeffects. One intrinsic loss due to the pumping process results from theso-called quantum defect. The quantum defect is defined as the ratio ofthe pump wavelength to the lasing wavelength, and as such, acts as ameasure of the amount of pump energy that is not carried by theamplified radiation, and thus is converted to excess energy within thefiber. Such energy can result in negative thermal effects or negativeradiation effects (e.g., spontaneously emitted photons escaping from thefiber and not contributing to amplification of the electromagneticradiation in the core). This can be particularly problematic in activeoptical fibers doped with elements having high quantum defect values.Another loss mechanism results from the emission of phonons (i.e.,lattice vibrations). Such phonons can lead to an increase in thermalenergy of the amplifier. In severe cases, the fiber itself canexperience degradation due to overwhelming heating because of theassociated energy dissipation effects. This is particularly the case forhigh peak power fiber amplifiers where the active doped fibers absorbthe pump energy over a relatively short distance in order to mitigatesubstantial adverse non-linear optical effects (e.g., stimulatedBrillouin scattering or stimulated Raman scattering).

Since a substantial fraction of the gain in an optical fiber amplifyingsystem usually takes place within the portion of the active opticalfiber that is nearest to the pump optical source (e.g., within a fewcentimeters of the end of the fiber coupled to the pump source), thethermal effects can be most pronounced in this region. This can beparticularly problematic near fusion splices, sometimes called “criticaljunctions”. Thus, the fusion splices can be the first section of anoptical fiber amplifying system to fail if the thermal effects are notadequately accounted for.

Therefore, in order for optical fiber amplifiers and lasers to maximizepower output, an effective means of handling the adverse energydissipation effects in such systems without adversely affecting otheraspects of the system, can be desired.

SUMMARY

The specification and drawings disclose embodiments that relate to afiber encapsulation mechanism for energy dissipation in a fiberamplifying system.

An example fiber encapsulation structure within an optical fiberamplifier may allow for energy dissipation. The optical fiber amplifiermay include an active, dual-clad optical fiber that is optically pumpedto amplify a signal. When amplifying the signal, the active, dual-cladoptical fiber may generate excess photons and excess heat. The fiberencapsulation structure may surround one or more sections of the active,dual-clad optical fiber. For example, the fiber encapsulation structuremay surround a section of the active, dual-clad optical fiber from whicha second cladding has been stripped. Further, the fiber encapsulationstructure may conduct the excess photons and the excess heat away fromthe optical fiber. In order to conduct the excess photons and the excessheat away from the optical fiber, the fiber encapsulation structure mayinclude a polymer layer, a heatsink layer adjacent to the polymer layer,and an optically transparent layer adjacent to the polymer layeropposite the heatsink layer.

In a first aspect, the disclosure describes an optical fiber amplifier.The optical fiber amplifier includes an optical fiber that includes again medium. The optical fiber amplifier also includes a polymer layerthat at least partially surrounds the optical fiber. The polymer layeris optically transparent. Further, the optical fiber amplifier includesa pump source configured to optically pump the optical fiber. Opticalpumping by the pump source amplifies optical signals in a wavelengthrange transmitted through the gain medium of the optical fiber andgenerates excess heat and excess photons. The optical fiber amplifieradditionally includes a heatsink layer disposed adjacent to the polymerlayer. The heatsink layer conducts the excess heat away from the opticalfiber. The optical fiber further includes an optically transparent layerdisposed adjacent to the polymer layer opposite the heatsink layer. Theoptically transparent layer transmits the excess photons away from theoptical fiber.

In a second aspect, the disclosure describes a method. The methodincludes optically pumping, by a pump source, an optical fiber thatincludes a gain medium. Optically pumping by the pump source amplifiesoptical signals in a wavelength range transmitted through the gainmedium of the optical fiber and generates excess heat and excessphotons. The method also includes transmitting, to a polymer layer thatat least partially surrounds the optical fiber, the excess heat and theexcess photons. The polymer layer is optically transparent. In addition,the method includes conducting, by a heatsink layer disposed adjacent tothe polymer, the excess heat away from the optical fiber. The methodfurther includes conducting, by an optically transparent layer disposedadjacent to the polymer layer opposite the heatsink layer, the excessphotons away from the optical fiber.

In a third aspect, the disclosure describes a method of assembling anoptical fiber amplifier. The method includes connecting an output end ofa pump source to an input end of an optical fiber. The optical fiberincludes a gain medium. The pump source is configured to optically pumpthe optical fiber to amplify optical signals in a wavelength rangetransmitted through the gain medium of the optical fiber. Opticallypumping the optical fiber to amplify optical signals generates excessheat and excess photons. The method also includes placing at least aportion of the optical fiber adjacent to a heatsink layer. The heatsinklayer conducts the excess heat away from the optical fiber. In addition,the method includes surrounding the portion of the optical fiberadjacent to the heatsink layer with a polymer layer. The polymer layeris optically transparent. Further, the method includes placing anoptically transparent layer adjacent to the polymer layer opposite theheatsink layer. The optically transparent layer transmits the excessphotons away from the optical fiber.

In an additional aspect, the disclosure describes a system. The systemincludes a means for optically pumping an optical fiber that includes again medium. The means for optically pumping amplifies optical signalsin a wavelength range transmitted through the gain medium of the opticalfiber and generates excess heat and excess photons. The system alsoincludes a means for transmitting, to a polymer layer that at leastpartially surrounds the optical fiber, the excess heat and the excessphotons. The polymer layer is optically transparent. In addition, thesystem includes a means for conducting the excess heat away from theoptical fiber. The means for conducting the excess heat away from theoptical fiber is disposed adjacent to the polymer. The system furtherincludes a means for conducting the excess photons away from the opticalfiber. The means for conducting the excess photons away from the opticalfiber is disposed adjacent to the polymer layer opposite the means forconducting the excess heat away from the optical fiber.

Additionally, the disclosure describes a system for assembling anoptical fiber amplifier. The system includes a means for connecting anoutput end of a pump source to an input end of an optical fiber. Theoptical fiber comprises a gain medium. The pump source is configured tooptically pump the optical fiber to amplify optical signals in awavelength range transmitted through the gain medium of the opticalfiber. Optically pumping the optical fiber to amplify optical signalsgenerates excess heat and excess photons. The system also includes ameans for placing at least a portion of the optical fiber adjacent to aheatsink layer. The heatsink layer conducts the excess heat away fromthe optical fiber. In addition, the system includes a means forsurrounding the portion of the optical fiber adjacent to the heatsinklayer with a polymer layer. The polymer layer is optically transparent.Further, the system includes a means for placing an opticallytransparent layer adjacent to the polymer layer opposite the heatsinklayer. The optically transparent layer transmits the excess photons awayfrom the optical fiber.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the figures and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a schematic of an optical fiber amplifier, according toexample embodiments.

FIG. 1B is a block diagram of a LIDAR system, according to exampleembodiments.

FIG. 2 is a cross-sectional illustration of a fiber encapsulationstructure, according to example embodiments.

FIG. 3 is an illustration of a connection between two optical fibers,according to example embodiments.

FIG. 4 is an illustration of a portion of an optical fiber amplifier,according to example embodiments.

FIG. 5 is a cross-sectional illustration of a fiber encapsulationstructure, according to example embodiments.

FIG. 6 is a flow chart illustration of a method, according to exampleembodiments.

FIG. 7 is a flow chart illustration of a method, according to exampleembodiments.

DETAILED DESCRIPTION

Example methods and systems are described herein. Any example embodimentor feature described herein is not necessarily to be construed aspreferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that certain aspects of the disclosed systemsand methods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

Furthermore, the particular arrangements shown in the figures should notbe viewed as limiting. It should be understood that other embodimentsmight include more or less of each element shown in a given figure. Inaddition, some of the illustrated elements may be combined or omitted.Similarly, an example embodiment may include elements that are notillustrated in the figures.

Still further, the terms “optical”, “optically”, etc., as used herein,are not meant to limit the embodiments to applications that use visiblewavelengths. In various embodiments, both explicitly and implicitlycontemplated herein, other wavelength ranges may be used. For example,electromagnetic radiation having an infrared wavelength (e.g., 1.55 μm)may be transmitted down an optical fiber, as described herein.

I. OVERVIEW

Example embodiments may relate to devices, systems, and methods forextracting excess energy from an optical fiber within an optical fiberamplifier (e.g., an optical fiber amplifier for use in a transmitter ofa light detection and ranging, LIDAR, system). For example, an active,dual-clad optical fiber that is part of the second gain stage of anoptical fiber amplifying system may generate excess energy in the formof heat (i.e., phonons) or light (i.e., photons) while amplifyingradiation in a core of the active, dual-clad optical fiber. This energymay be removed from the vicinity of the active, dual-clad optical fiberand radiated away to prevent degradation of the active, dual-cladoptical fiber/optical fiber amplifying system.

A specific example embodiment may include a fiber encapsulationstructure having an optical fiber submersed in an optically transparentpolymer. The polymer may be sandwiched between a heatsink (e.g., copper)and an optically transparent layer (e.g., glass). The fiberencapsulation structure may be fabricated according to a specificprocess. For example, the polymer may be liquefied, and then poured (orinjected) on top of the heatsink. The optical fiber may then be placedinto the polymer. Next, the optically transparent layer may bepositioned above and along the optically transparent polymer to define aheight of the optically transparent polymer region. The opticallytransparent polymer may then be allowed to cure. As the opticallytransparent polymer is cured, it may solidify, thus holding the opticalfiber in place and also adhering to and supporting the opticallytransparent layer.

In various embodiments, various subsections of the optical fiberamplifier may reside in the polymer region. For example, a portion of asecond stage of an optical fiber amplifying system (e.g., an active,dual-clad optical fiber) may reside in the polymer. Further, in someembodiments, only a portion of an optical fiber may reside in thepolymer (e.g., the section of the active, dual-clad optical fibernearest to a fusion splice). This may be a feature designed to accountfor the fact that a majority of the amplification, and thus a majorityof the adverse energy generation effects, may occur within a firstregion of the optical fiber.

In some embodiments, the active optical fiber within the opticallytransparent polymer may be an entirely dual-clad optical fiber. Forexample, the active optical fiber may guide electromagnetic radiation ofa signal wavelength in a core region and electromagnetic radiation of apump wavelength in a first cladding region. To do so, the optical fibermay be designed with a low refractive index second cladding to assist incontaining the pump radiation within the first cladding. In alternateembodiments, the active optical fiber may only include a singlecladding. In such embodiments, the polymer region may have a low enoughrefractive index to serve the purpose of the second cladding by ensuringthe pump light is efficiently guided within the first cladding of theactive optical fiber (i.e., the polymer region may act as a surrogatesecond cladding region based upon the polymer region's refractiveindex).

The polymer, itself, can be conductive to both thermal energy andphotonic energy. As excess energy is generated by the active opticalfiber (e.g., in a region immediately after the fusion splice where theabsorption of the pump energy is the highest), the excess energy isimparted to the polymer. The thermal energy eventually makes its way tothe heatsink, where it is extracted from the fiber encapsulationstructure, and the photonic energy eventually makes its way to theoptically transparent layer where it is radiated away from the fiberencapsulation structure. This may not occur directly, however. Forexample, if a photon is emitted from the active optical fiber comes intocontact with the heatsink, the photon may be reflected back to theinterior of the fiber encapsulation structure (e.g., back toward acenter of the polymer). The photon may be reflected by one of thesegments of the active optical fiber, again toward the heatsink. Thismay occur multiple times before the photon is eventually directed to theoptically transparent layer and radiated out of the fiber encapsulationstructure. To increase the thermal conductivity of the polymer, thepolymer may have materials embedded therein (e.g., the polymer may bedoped with other materials) that have a high thermal conductivity (e.g.,Boron Nitride, Alumina, Silica, or other minerals, including MgF₂ orCaF₂).

As the polymer region is typically amorphous, and defined by the shapeof the structures containing it as it cures, the polymer can take onvarious shapes. In some embodiments, the shape of the polymer will be aslab, defined between two other slabs (e.g., the optically transparentlayer and the heatsink). Alternatively, the polymer could be shapedcylindrically if defined between two half-cylindrical shells (e.g., anoptically transparent, half-cylindrical shell and a heatsink,half-cylindrical shell). Other shapes of the polymer are also possible.

II. EXAMPLE SYSTEMS

The following description and accompanying drawings will elucidatefeatures of various example embodiments. The embodiments provided are byway of example, and are not intended to be limiting. Specifically, it isunderstood that the scale and dimensions of various components may notreflect actual reductions to practice. For example, the optical fibersillustrated in the accompanying drawings may be many times larger (e.g.,with respect to the heatsink layer, the polymer layer, or the opticallytransparent layer) in the drawings than would reasonably be expected, soas to clearly illustrate the features of the optical fibers.

FIG. 1A is a schematic of an optical fiber amplifier 100, according toexample embodiments. The optical fiber amplifier 100 may containmultiple stages, as illustrated in FIG. 1A by the sections separated bydashed lines. The optical fiber amplifier 100 may include a seedingstage 110, a preamplifying stage 120, and a booster amplifying stage150. The seeding stage 110 may include a pulsed seed laser 112 and abuilt-in isolator 114. The preamplifying stage 120 may include asingle-mode pump diode 122, a wavelength-division multiplexer (WDM) 126,an active single-mode optical fiber 128, an amplified spontaneousemission (ASE) filter 132, an optical isolator 134, and a mode scrambler136. The booster amplifying stage 150 may include multimode pump lasers152, a multimode combiner 154, a fiber encapsulation structure 200, anactive, dual-clad optical fiber 156, and an end emitter 158. The variousstages and the various optical components within each stage of theoptical fiber amplifier 100 may be connected to one another at fusionsplices 102 (indicated in FIG. 1A by an “X” symbol). Other arrangementsof components, as well as additions or subtractions of components, maybe used in various embodiments.

FIG. 1B is a block diagram of a LIDAR system 190, according to exampleembodiments. The LIDAR system 190 includes a transmitter 192 and areceiver 194. The transmitter includes the optical fiber amplifier 100illustrated in FIG. 1A. The LIDAR system 190 may be used for navigationwithin an autonomous vehicle, for example.

In example embodiments, a signal may be emitted from the optical fiberamplifier 100 within the transmitter 192. The signal may be scattered byobjects within a scene and consequently detected by the receiver 194(e.g., by one or more light detectors within the receiver 194) of theLIDAR system 190 to analyze the scene (e.g., to determine the shape ofthe object or the object's distance from the LIDAR system 190). In someembodiments, the signal emitted from the optical fiber amplifier 100 mayhave a wavelength of between 880 nm and 920 nm (e.g., 900 nm for shortrange or midrange LIDAR applications). In alternate embodiments, thesignal emitted from the optical fiber amplifier 100 may have awavelength of between 1.525 μm and 1.565 μm (e.g., 1.55 μm for longrange LIDAR applications). In still other embodiments, the signalemitted from the optical fiber amplifier 100 may have a wavelength ofbetween 1.03 μm and 1.07 μm (e.g., 1.05 μm). Further, the power emittedby the optical fiber amplifier 100 may be below the maximum “eye safe”power limit for a Class 1 laser system (specified by the IEC 60825-1standard), in some embodiments. The receiver 194 may include componentsin addition to the light detector(s), such as lenses, stages, filters, acomputing device, etc.

Many of the components of the optical fiber amplifier 100 have opticalfibers (e.g., integrated or embedded optical fibers) as inputs oroutputs. Thus, the fusion splices 102 may provide a coupling mechanismfor these components. The fusion splices 102 may be created by heatingthe two optical fibers to be fused (e.g., using a gas flame, a laser, oran electric arc). Further, creation of the fusion splices 102 betweentwo, or more, optical fibers may include stripping one or multiple ofthe optical fibers, cleaning one or multiple of the optical fibers,cleaving one or multiple of the optical fibers, and splicing the opticalfibers (e.g., using a commercial fusion splicer).

The seeding stage 110 may be the stage of the optical fiber amplifier100 that generates the electromagnetic radiation that is to be amplifiedby the optical fiber amplifier 100. This electromagnetic radiation maybe referred to as the “seed radiation”, the “signal radiation” or the“output radiation”. Other names for this electromagnetic radiation arealso possible. A wavelength of the signal radiation may be selected by afiber amplifier designer to correspond with the amplification mechanismthat is provided at other stages of the optical fiber amplifier 100. Forexample, if later stages of the optical fiber amplifier 100 includeerbium-doped, dual-clad optical fibers as a gain mechanism foramplifying the signal radiation, the signal radiation may have awavelength between 1.525 μm and 1.565 μm (e.g., 1.55 μm).

The pulsed seed laser 112 may emit electromagnetic radiation having thewavelength of the signal radiation. In some embodiments, the pulsed seedlaser 112 may be an electrically pumped laser diode. The pulsed seedlaser 112 may be pulsed (e.g., having pulses between 1 ns and 10 ns induration). When compared with a continuous wave (CW) laser, the pulsedseed laser 112, and consequently the optical fiber amplifier 100, mayoutput a higher peak power while maintaining an equivalent averagepower. This may serve to partially mitigate excess energy emission(e.g., heat generation) that occurs within the active portions of theoptical fiber amplifier 100 (e.g., the booster amplifying stage 150).However, in alternate embodiments, a CW seed laser may instead be used.Additionally, in some embodiments, multiple seed lasers may be used inthe seeding stage 110 (e.g., two CW seed lasers). The pulsed seed laser112 may have the following emission specifications, in exampleembodiments: wavelength of 1542 nm; pulses of 3 ns duration occurring at150 kHz; and 0.1 W of peak power output, which corresponds to 45 μW ofcontinuous power output.

The built-in isolator 114 may be a component of the seeding stage 110,as illustrated In FIG. 1A. In alternate embodiments, the built-inisolator 114 may be a sub-component of the pulsed seed laser 112. Thebuilt-in isolator 114 may include non-reciprocal optics that only allowunidirectional transmission of light. As such, the built-in isolator 114may prevent feedback (e.g., reverse propagating electromagneticradiation, such as electromagnetic radiation that was reflected at oneof the fusion splices 102) from being transmitted to the pulsed seedlaser 112 from the rest of the optical fiber amplifier 100. Asillustrated, an output of the built-in isolator 114 may be connected toan input of a WDM 126 within the preamplifying stage 120.

The preamplifying stage 120 may be provided as a first of two gainstages of the optical fiber amplifier 100. The preamplifying stage 120may have a relatively low mode area when compared with the boosteramplifying stage 150. This may permit the preamplifying stage 120 toexperience higher gain efficiency than the booster amplifying stage 150.Further, the relatively low mode area may permit the preamplifying stage120 to incorporate single-mode fibers (such as the active single-modeoptical fiber 128). The active single-mode optical fiber 128 may haveenhanced optical guiding characteristics when compared with multi-modefibers. This may allow for tight coiling of the active single-modeoptical fiber 128, and thus a more compact physical arrangement of thepreamplifying stage 120 than the booster amplifying stage 150.

The single-mode pump diode 122 may be the pump source for thepreamplifying stage 120 of the optical fiber amplifier 100. Alternatepump sources (e.g., other optical fiber amplifiers or lasers) may beused instead of or in addition to the single-mode pump diode 122, invarious embodiments. The single-mode pump diode 122 may provide pumpradiation at a wavelength that corresponds to a gain medium being usedwithin the optical fiber amplifier 100 (e.g., within the preamplifyingstage 120 of the optical fiber amplifier 100) to provide gain for thetransmitted optical signals. For example, for a gain medium thatincludes erbium within the active single-mode optical fiber 128, thesingle-mode pump diode 122 may provide radiation having a wavelengthcentered on 980 nm or 1.48 μm. As illustrated, an output of thesingle-mode pump diode 122 may be connected to the WDM 126. Some of thebenefits of a decreased mode area presented with respect to thedescription of the preamplifying stage 120 may be enabled by the use ofthe single-mode pump diode 122. In some embodiments, the preamplifyingstage 120 may additionally or alternatively include a multimode source.

The WDM 126 is configured to take multiple inputs (e.g., two asillustrated in FIG. 1A), and multiplex them onto a single output. Forexample, the WDM 126 may take an input from the single-mode pump diode122 and another from the built-in isolator 114, and multiplex themtogether onto an output fiber that is connected to an active single-modeoptical fiber 128. In this case, the signal radiation and the pumpradiation co-propagate in the active single-mode optical fiber 128(i.e., the active single-mode optical fiber 128 is being pumped in aco-pumped configuration).

The active single-mode optical fiber 128 may be a silica fiber thatincludes a gain medium to provide amplification during transmission ofthe pump and the signal radiation down the active single-mode opticalfiber 128. For example, the active single-mode optical fiber 128 may bedoped with erbium, ytterbium, thulium, or other rare earth elements.Other gain media, including elements that are not rare earth elements,are also possible. The active single-mode optical fiber 128 may use suchdopants to amplify the electromagnetic radiation from the pulsed seedlaser 112. This may occur through the physical process of stimulatedemission. The active single-mode optical fiber 128 may have a coreregion surrounded by a cladding. Further, in some embodiments, theactive single-mode optical fiber 128 may be configured to permit onlythe fundamental transverse electromagnetic mode (i.e., TEM₀₀) topropagate down the active single-mode optical fiber 128. As illustrated,an output of the active single-mode optical fiber 128 may be connectedto an input of the ASE filter 132.

The ASE filter 132 may prevent transmission of ASE noise. For example,the ASE filter 132 may have a pass band that includes the wavelengthrange being amplified within the active single-mode optical fiber 128,but excludes the wavelength range of any ASE noise. In some embodiments,the mechanism for amplifying the seed radiation may be stimulatedemission. Thus, extraneous amplified spontaneous emission may hinder theperformance of the optical fiber amplifier 100. As such, the ASE filter132 may prevent such adverse effects. As illustrated, an output of theASE filter 132 may be connected to an input of the optical isolator 134.

The optical isolator 134 may serve a similar purpose to the built-inisolator 114, in example embodiments. Analogous to the built-in isolator114, the optical isolator 134 may also include non-reciprocal optics.The optical isolator 134 may prevent feedback (e.g., reverse propagatingelectromagnetic radiation, such as electromagnetic radiation that wasreflected at one of the fusion splices 102, or electromagnetic radiationemission from the booster amplifying stage 150 that is propagating inthe backward direction) from being transmitted back to the ASE filter132 or the active single-mode optical fiber 128. As illustrated, anoutput fiber of the optical isolator 134 may be connected to an inputfiber of the mode scrambler 136.

The mode scrambler 136 may convert the single-mode, or nearlysingle-mode, output of the optical isolator 134 into a multimode output.The mode scrambler 136 may include a step-graded-step (S-G-S)configuration of fibers, which is a set of a step-index profile fiber, agraded-index profile fiber, and another step-index profile fiberconnected in sequence (step-index and graded-index referring to therefractive index profile within the fiber from center to outer radius).Additionally or alternatively, the mode scrambler 136 may include astep-index fiber with bends, which is a step-index optical fiber havingmany bends of small radius (based on wavelength). As illustrated, anoutput fiber of the mode scrambler 136 may be connected to an inputfiber of the multimode combiner 154 of the booster amplifying stage 150.At the output of the mode scrambler 136, the waveform of electromagneticradiation may be an amplified/modified version of that output by thepulsed seed laser 112. For example, the output of the mode scrambler 136may have the following characteristics: wavelength of 1542 nm; pulses of2.5 ns duration occurring at 150 kHz; and 500 W of peak power output,which corresponds to 187.5 mW of continuous power output.

The booster amplifying stage 150 may be provided as a second of two gainstages of the optical fiber amplifier 100. The active, dual-clad opticalfiber 156 within the booster amplifying stage 150 may have a relativelyhigh mode area when compared with the active single-mode optical fiber128 within the preamplifying stage 120. This may permit the boosteramplifying stage 150 to experience reduced nonlinear optical effectswhen compared with alternate lower mode area fibers at equivalent powerdensities. Further, the relatively high mode area may permit the active,dual-clad optical fiber 156 within the booster amplifying stage 150 tohave an increased ratio of core to cladding than would otherwise bepossible, thereby improving pump absorption, and decreasing the lengthof fiber that is used to obtain a given amount of amplification.

The multimode pump lasers 152 may be the pump sources for the boosteramplifying stage 150 of the optical fiber amplifier 100. Alternate pumpsources (e.g., other optical fiber amplifiers or lasers) may be usedinstead of or in addition to the multimode pump lasers 152, in variousalternate embodiments. The multimode pump lasers 152 may provide pumpradiation at a wavelength that corresponds to a gain medium within thebooster amplifying stage 150 (e.g., within the active, dual-clad opticalfiber 156). For example, if the gain medium includes erbium andytterbium, the multimode pump lasers 152 may provide pump radiationhaving a wavelength centered between 920 nm and 980 nm (e.g., 950 nm).As illustrated, output fibers of the multimode pump lasers 152 may beconnected to input fibers of the multimode combiner 154. Some of thebenefits of an increased mode area presented with respect to thedescription of the booster amplifying stage 150 may be enabled by theuse of the multimode pump lasers 152 (in tandem with the multimodeoutput from the mode scrambler 136).

The multimode combiner 154 may combine inputs from the multimode pumplasers 152 and an input from the mode scrambler 136 into a single outputfiber. As in the embodiment of FIG. 1A, the multimode combiner 154 maybe a (2+1)×1 combiner. Further, as illustrated, the multimode pumplasers 152 and the fiber output from the mode scrambler 136 may befusion spliced onto exposed fibers of the multimode combiner 154.Alternatively, fiber pigtails may be used to adjoin the multimode pumplasers or the fiber output from the mode scrambler to the multimodecombiner. In alternate embodiments of the booster amplifying stage,where there are more multimode pump lasers, the multimode combiner mayhave a greater number of input ports. As illustrated, an output of themultimode combiner 154 may be connected to an input of the active,dual-clad optical fiber 156. In some embodiments, the output of themultimode combiner 154 may be a dual-clad optical fiber having the pumpradiation from the multimode pump lasers 152 confined to a claddingregion and the signal radiation from the output of the mode scrambler136 confined to a core region. For example, the dual-clad optical fiberoutput may be a passive optical fiber (i.e., an optical fiber not dopedwith a gain medium).

The fiber encapsulation structure 200 may be used to alleviate excessenergy generation effects (e.g., excess photons or excess heat) thatarise when optically pumping the active, dual-clad optical fiber 156(i.e., when the active, dual-clad optical fiber 156 is amplifying signalradiation). The fiber encapsulation structure 200 may surround theactive, dual-clad optical fiber 156 for portions of the active,dual-clad optical fiber 156. In some embodiments, the fiberencapsulation structure may surround the active, dual-clad optical fiberfor the entire length of the active, dual-clad optical fiber.Additionally, the fiber encapsulation structure 200 may surround thefusion splice 102 between the output fiber of the multimode combiner 154and the active, dual-clad optical fiber 156 (e.g., the fusion splice 102may be embedded within the polymer layer 206). The fiber encapsulationstructure 200 is described in further detail with respect to FIG. 2.

The active, dual-clad optical fiber 156 may be the region of the opticalfiber amplifier 100 where a majority of the amplification occurs. Forexample, the active, dual-clad optical fiber 156 may have a core thatincludes a gain medium (e.g., doped with a rare earth element, such aserbium, ytterbium, or thulium) that amplifies a signal in the corethrough the process of stimulated emission. In order to achieve gain,pump radiation may be guided in a first cladding region, and used toexcite the dopants. A second cladding may be included in the active,dual-clad optical fiber 156 to contain the pump radiation within thefirst cladding. For example, the refractive index profile within theactive, dual-clad optical fiber 156 may defined such that the signalradiation is substantially confined to the core and the pump radiationis substantially confined to the first cladding and the core (e.g., therefractive index of the core is greater than the refractive index of thefirst cladding, which are both greater than the refractive index of thesecond cladding). The second cladding may be able to withstandtemperatures up to 125 degrees Celsius, in some embodiments. The secondcladding may include acrylate doped with fluorine, in variousembodiments. In an example embodiment, the active, dual-clad opticalfiber 156 may have an erbium/ytterbium-doped core, a first claddinglayer of fused silica (having a refractive index between 1.44 and 1.46),and a second cladding layer of a low-index polymer (having a refractiveindex between 1.37 and 1.4).

The amplification in the active, dual-clad optical fiber 156 can lead topeak power densities that are large enough to induce substantialnon-linear optical effects. The non-linear optical effects may include,for example, self-phase modulation (SPM), stimulated Brillouinscattering (SBS), or stimulated Raman scattering (SRS). Many non-linearoptical effects have a strong dependence on interaction length (i.e.,the fiber length over which the pump radiation and the signal radiationinteract to stimulate amplification). As such, to mitigate non-linearoptical effects, the active, dual-clad optical fiber 156 may have anoptimized length (i.e., the length of the active, dual-clad opticalfiber 156 may be as short as possible while achieving the desired amountof amplification). For example, the active, dual-clad optical fiber 156may be 2 meters in length or less.

In some embodiments, the active, dual-clad optical fiber 156 mayadditionally include a jacket, surrounding the second cladding, tomechanically protect the fiber from wear. Further, the core, the firstcladding, and the second cladding may not be concentric to one another.By breaking the circular symmetry (i.e., by not having the componentsplaced concentric to one another), there may be a greater overlap ofmodes between the pump radiation and the signal radiation, therebyincreasing amplification over the same relative distance. Even further,the sections of the active, dual-clad optical fiber 156 (i.e., the core,the first cladding, and the second cladding) may not have circularshapes, in various embodiments. For example, in some embodiments, thefirst cladding may have a hexagonal shape to increase reflections ofpump radiation toward the core. In alternate embodiments, the active,dual-clad optical fiber may be partially or wholly replaced with anothertype of gain fiber (e.g., a fiber having only a single cladding).

As illustrated, an output of the active, dual-clad optical fiber 156 maybe connected to an input of the end emitter 158. Also as illustrated,the input of the active, dual-clad optical fiber may be connected to theoutput fiber of the multimode combiner 154. The pump radiation coupledto the first cladding of the active, dual-clad optical fiber 156 may betransmitted to the active, dual-clad optical fiber 156 from the claddingof the output fiber of the multimode combiner 154. Further, the signalradiation coupled to the core of the active, dual-clad optical fiber 156may be transmitted to the active, dual-clad optical fiber 156 from thecore of the output fiber of the multimode combiner 154. To form aneffective fusion splice between the output fiber of the multimodecombiner 154 and the input of the active, dual-clad optical fiber 156,the second cladding, and any jacket surrounding the second cladding, ofthe active, dual-clad optical fiber 156 may be stripped from the active,dual-clad optical fiber 156 prior to creating the fusion splice 102.

The end emitter 158 provides a termination to the active, dual-cladoptical fiber 156. As such, the end emitter 158 further provides atermination to the optical fiber amplifier 100. As illustrated, the endemitter 158 may transmit electromagnetic radiation (e.g., radiation ofthe signal wavelength) away from the optical fiber amplifier 100. Theend emitter 158 may include an end-cap at the end of the fiber to expandthe electromagnetic radiation beam prior to the beam exiting from theactive, dual-clad optical fiber 156 into free space. In someembodiments, the end emitter may include additional free space optics toradiate the signal. In such embodiments, the end emitter may include alens or a mirror. The electromagnetic radiation emitted from the endemitter 158 may have a high average power (e.g., between 1 W and 20 W)and a high peak power (e.g., between 1 kW and 100 kW) relative to thesignal power provided by the pulsed seed laser 112.

FIG. 2 is a cross-sectional illustration of the fiber encapsulationstructure 200 that is illustrated in FIG. 1A. The fiber encapsulationstructure 200 may encapsulate segments of the core 201 and firstcladding 202 of the active, dual-clad optical fiber 156, where thesecond cladding has been stripped from the active, dual-clad opticalfiber 156. For example, the segments of the core 201 and the firstcladding 202 may be from multiple windings of the active, dual-cladoptical fiber 156. Further, the fiber encapsulation structure 200 mayinclude an optically transparent layer 204, a polymer layer 206, and aheatsink layer 208. Also illustrated in FIG. 2 are photons 212 (e.g.,photons having a wavelength of 1 μm) and heat 214, both emitted from thegain medium (e.g., rare earth element, such as erbium or ytterbium)within the core 202 when the active, dual-clad optical fiber 156 isamplifying signal radiation (e.g., by being optically pumped by themultimode pump lasers 152). In alternate embodiments, the portion of theactive, dual-clad optical fiber that is surrounded by the fiberencapsulation structure may not be stripped of the second cladding. Forexample, if the second cladding of the active, dual-clad optical fiberhas a higher thermal conductivity than the polymer layer, the secondcladding may not be stripped from the active, dual-clad optical fiber inthe region that is surrounded by the polymer layer.

The scale of the components relative to one another, as illustrated inFIG. 2, is by way of example, and is not meant to necessarily representactual reductions to practice. For example, the core 201 and the firstcladding 202 may be smaller, in practice, relative to the opticallytransparent layer 204, the polymer layer 206, or the heatsink layer 208than illustrated. Further, in some embodiments, there may only be onestrand of the active, dual-clad optical fiber surrounded by the fiberencapsulation structure (as opposed to six windings, as illustrated inFIG. 2). Alternatively, there could be more windings surrounded by thefiber encapsulation structure than illustrated in FIG. 2. In still otherembodiments, the fiber encapsulation structure illustrated in FIG. 2 mayinstead surround a fiber that is not a component of an optical fiberamplifier. For example, an optical fiber used in a transmission linethat transmits signals having high peak powers may be surrounded by thefiber encapsulation structure.

The photons 212 and the heat 214 can produce adverse effects on the core201 and the first cladding 202 if not adequately accounted for. Forexample, if the photons 212 and the heat 214 are not transmitted awayfrom the active, dual-clad optical fiber 156, of which the core 201 andthe first cladding 202 are a part, the fiber could end up melting andultimately failing. The photons 212 and the heat 214 generated may bethe result of the quantum defect of the gain medium being used toamplify the signal radiation in the core 201.

In order to account for the photons 212 and the heat 214 emitted by thegain medium, the fiber encapsulation structure 200 may be used tosurround the core 201 and the first cladding 202. Adjacent to the firstcladding 202 may be the polymer layer 206. The polymer layer 206 may bean optically transparent polymer (e.g., a silicone polymer). Further,the polymer layer 206 may have a greater thermal conductivity or thermalstability than the second cladding of the active, dual-clad opticalfiber 156. For example, the polymer layer 206 may be able to withstandtemperatures up to 200 degrees Celsius without melting or degrading. Assuch, the polymer layer is designed to conduct the photons 212 and theheat 214 generated from the quantum defect away from the core 201 andthe first cladding 202. In some embodiments, the polymer layer 206 maybe formed as a liquid and poured (or casted, injected, or molded) on theheatsink layer 208 and then allowed to cure in the form of a solid.Further, at the edges of the polymer layer 206 (i.e., the sides of thepolymer layer 206 that are exposed to the exterior of the fiberencapsulation structure 200), the photons 212 may be transmitted awayfrom the fiber encapsulation structure 200. In addition, the polymerlayer 206 may have a refractive index lower than the refractive index ofthe first cladding 202 (e.g., a refractive index between 1.37 and 1.40).Thus, the pump radiation can still be effectively guided in the firstcladding 202 of the active, dual-clad optical fiber 156, with thepolymer layer 206 serving as a surrogate second cladding. In someembodiments, the polymer layer could replace the second cladding alongthe entire length of the active, dual-clad optical fiber.

Adjacent to the polymer layer 206, on one side, may be the heatsinklayer 208. In some embodiments, such as the embodiment of FIG. 2, theheatsink layer 208 may be a slab of a material, such as a metal, thathas a high thermal conductivity (e.g., copper). In alternateembodiments, the heatsink layer may be shaped as a half cylinder or atrough in which the core, the first cladding, and a portion of thepolymer layer reside. As illustrated, the heatsink layer 208 transmitsthe heat 214 away from the polymer layer 206 and, thus, away from thecore 201 and the first cladding 202. In some embodiments, the heatsinklayer 208 may be opaque (i.e., impermeable to the photons 212 emitteddue to the quantum defect). In such embodiments, the photons 212 may bereflected, or alternatively absorbed, by the heatsink layer 208.

Adjacent to the polymer layer 206, on a side opposite the heatsink layer208, may be the optically transparent layer 204. The opticallytransparent layer 204 may be transparent to a range of wavelengths,including the wavelengths of the photons 212 (e.g., infraredwavelengths, such as 1 μm). The optically transparent layer 204 maytransmit the photons 212 away from the polymer layer 206, and thus, awayfrom the core 201 and the first cladding 202. The optically transparentlayer 204 may be glass, in some embodiments.

Further, the optically transparent layer 204 may define the minimumdistance away from the core 201 and the first cladding 202 at which thephotons 212 can be absorbed. For example, if there is dust or otherparticles on a top surface of the optically transparent layer 204, theseparticles may absorb the photons 212. When the photons 212 are absorbedin such a way, heat may be produced. However, the heat may be producedat a sufficient distance, based on the location of the opticallytransparent layer 204, away from the core 201 and the first cladding 202so it does not adversely affect the performance of the active, dual-cladoptical fiber 156. Even further, in some embodiments, there may be anabsorptive material placed on the top surface of the opticallytransparent material (e.g., paint applied to the top surface) tointentionally absorb the photons.

In addition, the optically transparent layer 204 may provide the fiberencapsulation structure 200 a physical barrier to mechanically protectthe polymer layer 206. As illustrated, the heat 214 may not be readilytransmitted by the optically transparent layer 204 like the photons 212.As with the heatsink layer 208, the shape of the optically transparentlayer may also be different among various embodiments.

In some embodiments, the optically transparent layer 206 may bepositioned at a certain location adjacent to the polymer layer 206during a fabrication process while the polymer layer 206 is in a liquidform. This may define the height of the polymer layer 206 (e.g., if thepolymer layer 206 is injected between the optically transparent layer204 and the heatsink layer 208) and the position of the opticallytransparent layer 204. Then, as the polymer layer 206 begins to cure,the polymer layer 206 may solidify and adhere to the opticallytransparent layer 204. Other fabrication techniques are also possible.

FIG. 3 is an illustration of a connection 300 between two opticalfibers, according to example embodiments. The left optical fiber 302 maybe the output fiber of the multimode combiner 154 illustrated in FIG.1A. The right optical fiber may be the active, dual-clad optical fiber156 illustrated in FIG. 1A with the core 201 and the first cladding 202revealed when the active, dual-clad optical fiber 156 is partiallystripped of a second cladding 312. As indicated by the dashed line, thetwo fibers may be adjoined at a fusion splice 102. In order to fuse thetwo fibers at the fusion splice 102, the fibers may be heated (e.g.,using a gas flame, a laser, or an electric arc).

In some embodiments, the left optical fiber may be a dual-clad opticalfiber. In such embodiments, before adjoining the fibers at the fusionsplice, a second cladding (e.g., a polymer) may also be stripped fromthe left optical fiber.

FIG. 4 is an illustration of a portion 400 of an optical fiberamplifier, such as the optical fiber amplifier 100 of FIG. 1A. Theportion 400 of the optical fiber amplifier may include the fiberencapsulation structure 200 illustrated in FIG. 2. As in FIG. 2, thefiber encapsulation structure 200 may include an optically transparentlayer 204, a polymer layer 206, and a heatsink layer 208. Asillustrated, the fiber encapsulation structure 200 may encapsulate thecore 201 and the first cladding 202 of one or more windings of theactive optical fiber 156, where the active optical fiber 156 has itssecond cladding partially stripped. The fiber encapsulation structure200 may be disposed just beyond a fusion splice 102 (e.g., the fusionsplice 102 of the two adjoined fibers illustrated in FIG. 3). The regionjust beyond the fusion splice 102 may be the location of the opticalfiber amplifier 100 where signal radiation experiences the greatestamount of gain in the active optical fiber 156. Therefore, the regionjust beyond the fusion splice 102 may be the region with the greatestgeneration of excess heat and excess photons.

In alternate embodiments, an increased length of the active, dual-cladoptical fiber may be surrounded by the fiber encapsulation structure.For example, a larger portion of the active, dual-clad optical fiber maybe stripped of the second cladding, and multiple segments of thestripped, active, dual-clad optical fiber may be wound back and forthwithin the fiber encapsulation structure (similar to the cross-sectionalillustration of FIG. 2). Further, in some embodiments, the fusion spliceand portions of the output fiber of the multimode combiner may besurrounded by the fiber encapsulation structure. As stated previously,the size of the fibers relative to the fiber encapsulation structure 200illustrated in FIG. 4 may be of different proportions than those used inactual reductions to practice.

The portion 400 of the optical fiber amplifier may be fabricated using afabrication method. The method may include stripping the active,dual-clad optical fiber 156 of its second cladding, at least partially.The remaining core 201 and first cladding 202 may then be placedadjacent to an output fiber from a pre-amplifying stage of the opticalamplifier (similar to the illustration of FIG. 3). In addition, the twoadjacent fibers may be sufficiently heated to fuse to one another,thereby forming the fusion splice 102. Further, the core 201 and thefirst cladding 202 nearest to the fusion splice 102 may then be placedon the heatsink layer 208. The polymer layer 206 may then be heated andpoured (or injected), in a liquid form, over the core 201 and the firstcladding 202, as well as on the heatsink layer 208. The polymer layer206 may fully engulf the core 201 and the first cladding 202, in someembodiments. The optically transparent layer 204 may then be suspendedover the polymer layer 206. This suspension may happen at a height thatis sufficient to ensure that the optically transparent layer 204 is notin physical contact with the first cladding 202. The polymer layer 206may then be allowed to solidify (i.e., causing the polymer layer 206 toundergo a phase change from a liquid to a solid). In some embodiments,the optically transparent layer 204 may instead be placed over theheatsink layer 208, the core 201, and the first cladding 202 prior topouring (or injecting) the polymer layer 206 in between the opticallytransparent layer 204 and the heatsink layer 208.

FIG. 5 is a cross-sectional illustration of a fiber encapsulationstructure 500. The fiber encapsulation structure 500 may encapsulatesegments of the core 201 and the first cladding 202 of the active,dual-clad optical fiber 156 from which the second cladding has beenstripped. For example, the segments of the core 201 and the firstcladding 202 may be multiple windings of the active, dual-clad opticalfiber 156. As in the fiber encapsulation structure 200 illustrated inFIG. 2, the fiber encapsulation structure 500 may include an opticallytransparent layer 204, a polymer layer 206, and a heatsink layer 208.The fiber encapsulation structure 500 may additionally include athermally conductive dopant 502. The components that are analogous tothe components of FIG. 2 (i.e., the core 201, the first cladding 202,the optically transparent layer 204, the polymer layer 206, and theheatsink layer 208) may be similarly described as in FIG. 2.

The thermally conductive dopant 502 may increase the thermalconductivity of the polymer layer 206. The thermally conductive dopant502 may also increase the thermal capacity of the polymer layer 206without substantially decreasing the optical transparency of the polymerlayer 206. Further, the thermally conductive dopant 502 may increase therate at which the polymer layer 206 transfers heat from the core 201 andthe first cladding 202 to the heatsink layer 208. In some embodiments,the thermally conductive dopant may include finely crushed CalciumFluoride (CaF₂) or another binary salt of fluoride (e.g., MgF₂ or BaF₂).Additionally or alternatively, the second cladding of the active,dual-clad optical fiber 156 may be doped with a thermally conductivedopant (e.g., CaF₂) to increase thermal conductivity.

III. EXAMPLE PROCESSES

FIGS. 6 and 7 are flowchart illustrations of methods, in accordance withexample embodiments. The methods described may include one or moreoperations, functions, or actions as illustrated by one or more of theillustrated blocks. Although the blocks are illustrated in a sequentialorder, these blocks may in some instances be performed in parallel, orin a different order than those described herein. Also, the variousblocks may be combined into fewer blocks, divided into additionalblocks, or removed based upon the desired implementation. Further,additional blocks describing additional, non-essential steps may beincluded in some variations of the methods contemplated herein.

FIG. 6 is a flow chart illustration of a method 600 of amplifying lightusing an optical fiber amplifier, according to example embodiments.

At block 602, the method 600 includes optically pumping, by a pumpsource, an optical fiber that includes a gain medium (e.g., the active,dual-clad optical fiber 156 of FIG. 1A). Optically pumping by the pumpsource amplifies optical signals in a wavelength range transmittedthrough the gain medium of the optical fiber and generates excess heatand excess photons.

Block 602 may also include providing the optical signals in thewavelength range transmitted through the gain medium. For example, theoptical signals may be provided by a seeding stage (e.g., the seedingstage 110 of the optical fiber amplifier 100 illustrated in FIG. 1A) andamplified by a preamplifying stage (e.g., the preamplifying stage 120 ofthe optical fiber amplifier 100 illustrated in FIG. 1A).

Block 602 may further include pumping the optical fiber that includesthe gain medium using one or more multimode pump lasers (e.g., themultimode pump lasers 152 illustrated in FIG. 1A).

At block 604, the method 600 includes transmitting, to a polymer layer(e.g., the polymer layer 206 illustrated in FIG. 2) that at leastpartially surrounds the optical fiber, the excess heat and the excessphotons. The polymer layer is optically transparent.

At block 606, the method 600 includes conducting, by a heatsink layer(e.g., the heatsink layer 208 illustrated in FIG. 2) disposed adjacentto the polymer, the excess heat away from the optical fiber.

At step 608, the method 600 includes conducting, by an opticallytransparent layer (e.g., the optically transparent layer 204) disposedadjacent to the polymer layer opposite the heatsink layer, the excessphotons away from the optical fiber.

FIG. 7 is a flow chart illustration of a method 700 of fabricating anoptical fiber amplifier (e.g., the optical fiber amplifier 100illustrated in FIG. 1A or the fiber encapsulation structure 200illustrated in FIG. 2), according to example embodiments.

At block 702, the method 700 includes connecting an output end of a pumpsource to an input end of an optical fiber (e.g., the active, dual-cladoptical fiber 156 illustrated in FIG. 1A). The pump source is configuredto optically pump the optical fiber to amplify optical signals in awavelength range transmitted through the gain medium of the opticalfiber (e.g., within an active core of the optical fiber). Opticallypumping the optical fiber to amplify optical signals generates excessheat and excess photons. In some embodiments, the pump source mayinclude multiple multimode pump lasers (e.g., the multimode pump lasers152 illustrated in FIG. 1A).

At block 704, the method 700 includes placing at least a portion of theoptical fiber adjacent to a heatsink layer (e.g., the heatsink layer 208illustrated in FIG. 2). The heatsink layer conducts the excess heat awayfrom the optical fiber.

At block 706, the method 700 includes surrounding the portion of theoptical fiber adjacent to the heatsink layer with a polymer layer (e.g.,the polymer layer 206 illustrated in FIG. 2). The polymer layer isoptically transparent. Surrounding the portion of the optical fiber withthe polymer layer may include, in some embodiments, pouring (orinjecting) the polymer layer in a liquid form onto the heatsink layer,such that the polymer layer surrounds the optical fiber.

At block 708, the method 700 includes placing an optically transparentlayer (e.g., the optically transparent layer 204) adjacent to thepolymer layer opposite the heatsink layer. The optically transparentlayer transmits the excess photons away from the optical fiber.

In some embodiments, the method 700 may further include curing thepolymer layer so the polymer layer undergoes a phase change from aliquid to a solid and adheres to the optical fiber, the heatsink layer,and the optically transparent layer.

IV. CONCLUSION

The above detailed description describes various features and functionsof the disclosed systems, devices, and methods with reference to theaccompanying figures. While various aspects and embodiments have beendisclosed herein, other aspects and embodiments will be apparent. Thevarious aspects and embodiments disclosed herein are for purposes ofillustration only and are not intended to be limiting, with the truescope being indicated by the following claims.

What is claimed:
 1. A multi-stage optical fiber amplifier, comprising: apreamplifying stage, comprising: a first optical fiber comprising a gainmedium; and a first pump source arranged to optically pump the firstoptical fiber, wherein optically pumping the first optical fiberamplifies optical signals in a wavelength range transmitted through thegain medium of the first optical fiber; and a booster amplifying stage,comprising: a second optical fiber comprising a gain medium, wherein thesecond optical fiber is optically coupled to the first optical fiber andarranged to receive optical signals from the first optical fiber; apolymer layer that at least partially surrounds the second opticalfiber, wherein the polymer layer is optically transparent; a second pumpsource arranged to optically pump the second optical fiber, whereinoptically pumping the second optical fiber amplifies optical signals inthe wavelength range transmitted through the gain medium of the secondoptical fiber and generates excess heat and excess photons; a heatsinklayer disposed adjacent to the polymer layer, wherein the heatsink layerconducts the excess heat away from the second optical fiber; and anoptically transparent layer disposed adjacent to the polymer layeropposite the heatsink layer, wherein the optically transparent layertransmits the excess photons away from the second optical fiber.
 2. Themulti-stage optical fiber amplifier of claim 1, wherein the secondoptical fiber comprises a core that includes the gain medium and acladding layer that surrounds the core.
 3. The multi-stage optical fiberamplifier of claim 2, wherein at least a portion of the second opticalfiber comprises a second cladding layer that surrounds the claddinglayer, and wherein the second optical fiber is less than 2 meters inlength.
 4. The multi-stage optical fiber amplifier of claim 1, whereinthe gain medium of the second optical fiber comprises erbium orytterbium.
 5. The multi-stage optical fiber amplifier of claim 1,wherein the wavelength range includes 1.55 μm, 1.05 μm, or 900 nm. 6.The multi-stage optical fiber amplifier of claim 5, wherein 900 nm isincluded in the wavelength range for short range or midrange lightdetection and ranging (LIDAR) applications or 1.55 μm is included in thewavelength range for long range LIDAR applications.
 7. The multi-stageoptical fiber amplifier of claim 1, wherein the polymer layer comprisesa polymer material and an additional material, wherein the additionalmaterial increases a thermal conductivity of the polymer layer.
 8. Themulti-stage optical fiber amplifier of claim 7, wherein the additionalmaterial comprises a binary salt of fluoride.
 9. The multi-stage opticalfiber amplifier of claim 7, wherein the additional material does notsubstantially decrease the optical transparency of the polymer layer.10. The multi-stage optical fiber amplifier of claim 1, furthercomprising a third optical fiber, wherein a first end of the thirdoptical fiber is joined to the first optical fiber, and wherein a secondend of the third optical fiber is joined to the second optical fiber ata fusion splice.
 11. The multi-stage optical fiber amplifier of claim10, wherein the polymer layer surrounds the fusion splice.
 12. Themulti-stage optical fiber amplifier of claim 1, wherein the polymerlayer can withstand temperatures up to 200 degrees Celsius.
 13. Themulti-stage optical fiber amplifier of claim 1, wherein the multi-stageoptical fiber amplifier is a two-stage optical fiber amplifier.
 14. Themulti-stage optical fiber amplifier of claim 1, wherein the opticalfiber amplifier is a transmitting component of a light detection andranging (LIDAR) system, and wherein the optical signals are scattered byobjects within a scene and detected by a receiver of the LIDAR system toanalyze the scene.
 15. The multi-stage optical fiber amplifier of claim1, wherein the second optical fiber is less than 2 meters in length. 16.The multi-stage optical fiber amplifier of claim 1, wherein theoptically transparent layer further defines a minimum distance from thesecond optical fiber at which the excess photons may be absorbed. 17.The multi-stage optical fiber amplifier of claim 1, wherein theoptically transparent layer further mechanically protects the polymerlayer from damage.
 18. The multi-stage optical fiber amplifier of claim1, further comprising a pulsed seed laser, wherein the pulsed seed laserseeds the first optical fiber with the optical signals in the wavelengthrange.
 19. A method, comprising: optically pumping, by a first pumpsource, a first optical fiber comprising a gain medium, whereinoptically pumping the first optical fiber amplifies optical signals in awavelength range transmitted through the gain medium of the firstoptical fiber, and wherein the first pump source and the first opticalfiber are components of a preamplifying stage; transmitting, to a secondoptical fiber comprising a gain medium, amplified optical signals fromthe first optical fiber; optically pumping, by a second pump source, thesecond optical fiber, wherein optically pumping the second optical fiberamplifies optical signals in the wavelength range transmitted throughthe gain medium of the second optical fiber, and wherein opticallypumping the second optical fiber generates excess heat and excessphotons; transmitting, to a polymer layer that at least partiallysurrounds the second optical fiber, the excess heat and the excessphotons, wherein the polymer layer is optically transparent; conducting,by a heatsink layer disposed adjacent to the polymer layer, the excessheat away from the second optical fiber; and transmitting, by anoptically transparent layer disposed adjacent to the polymer layeropposite the heatsink layer, the excess photons away from the secondoptical fiber, wherein the second optical fiber, the second pump source,the polymer layer, the heatsink layer, and the optically transparentlayer are components of a booster amplifying stage.
 20. A lightdetection and ranging (LIDAR) system, comprising: a transmitterconfigured to emit a signal, wherein the transmitter comprises amulti-stage optical fiber amplifier, and wherein the multi-stage opticalfiber amplifier comprises: a preamplifying stage, comprising: a firstoptical fiber comprising a gain medium; and a first pump source arrangedto optically pump the first optical fiber, wherein optically pumping thefirst optical fiber amplifies optical signals in a wavelength rangetransmitted through the gain medium of the first optical fiber; and abooster amplifying stage, comprising: a second optical fiber comprisinga gain medium, wherein the second optical fiber is optically coupled tothe first optical fiber and arranged to receive optical signals from thefirst optical fiber; a polymer layer that at least partially surroundsthe second optical fiber, wherein the polymer layer is opticallytransparent; a second pump source arranged to optically pump the secondoptical fiber, wherein optically pumping the second optical fiberamplifies optical signals in the wavelength range transmitted throughthe gain medium of the second optical fiber and generates excess heatand excess photons; a heatsink layer disposed adjacent to the polymerlayer, wherein the heatsink layer conducts the excess heat away from thesecond optical fiber; and an optically transparent layer disposedadjacent to the polymer layer opposite the heatsink layer, wherein theoptically transparent layer transmits the excess photons away from thesecond optical fiber; and a receiver configured to receive the signalscattered by objects within a scene.